1. Field of the Invention
The present invention relates to semiconductor devices and, more particularly, to a mesa type semiconductor device.
2. Description of the Prior Art
In a semiconductor device such as a transistor, water or moisture or any other contaminant may adhere directly to the semiconductor surface and give rise to a degradation in the characteristics of the device. It has been well known that, in order to prevent such a degradation, a protective film may be provided on the semiconductor surface. Even in a semiconductor device thus provided with a protective film, however, there have still arisen degradations in the characteristics such as an increase in leakage current and a decrease in the breakdown voltage of a P-N junction. As the result of various studies on the cause of the degradations, the following has been discovered. When any contaminant adheres to the protective film, it becomes ionized under the influence of a fringe electric field during the operation of the semiconductor device. The ions promote the formation of a depletion layer or a layer of an inverted conductivity type at that surface of a semiconductor region to which the end of a P-N junction extends. The depletion layer or the inversion layer extends to that part of the semiconductor surface which is not covered with the protective film and at a portion which contacts the base electrode, so that a large leakage current is generated.
The generation of the large leakage current leads to the occurrence of devices having nonconforming electrical characteristics. For example, it brings about an increase in I.sub.CBO (the collector-base reverse current of the transistor for an open emitter) during a high-voltage reverse bias test and a performance test of the semiconductor device.